(a) Field of the Invention
This invention relates to a position control device for effecting a relative position change of an optical reproduction system with respect to a disc in a compact disc player of the compact disc digital audio system and, more particularly, to a position control device effecting such relative position change of the optical reproduction system on the basis of a Q subcode containing time information in the data format of the compact disc digital audio system.
(b) Description of the Prior Art
As shown in FIG. 1(a), the data format of the compact disc digital audio system is such that one (1) frame consists of 588 channel bits including a one-symbol (eight bits) subcode area. The subcode, completed using 98 data frames, is constituted of eight 98-bit channels P, Q, . . . , W, as shown in FIG. 1(b). The subcode in the channel Q (Q subcode) is composed as shown in FIG. 1(c). signal in the Q subcode has three modes 1, 2, and 3. The mode is determined by a preceding address signal.
The Q subcode in the mode 1 is time data in which are recorded the time which has elapsed from the beginning of playing of a certain music piece in the disc (referred to as "piece time" below) and the time which has elapsed from the beginning of playing of the first piece in the disc (referred to as "accumulated time" below). This time data is represented in the BCD code in respect of minutes, seconds and frames each consisting of 2 digits (since 1 digit is expressed by 4 bits, time data consists of 6 digits.times.4 bits=24 bits). The term "frame" in the time data represents a unit under one second (For distinguishing this "frame" representing a unit of time from the above mentioned "frame" representing a section of 588 channel bits, the "frame" representing a section of 588 channel bits will be referred to as "data frame" in the following description). Since 75 subcodes are obtained per second (transmission rate 4.3218 M bits/S.div.588 bits.div.98 frames =75), frames are represented by quinary septuagesimal representation, i.e., 75 frames constitute 1 second. Time data of Q subcode is displayed directly for indicating the reproduced position and also is used for detecting difference from a target address and thereby moving the optical system in a search operation such as random access. In the search operation, if time data of Q subcode can be always detected without fail, time difference data between present time and target time is always available and can be given as a speed command for the drive system and the control can be performed such that the time difference will be reduced to zero.
In the actual search operation, however, data is picked up from the disc while the optical head is being displaced with a result that failure in reading the time data of Q subcode sometimes takes place. In the case of such failure, control is performed holding the Q subcode which has been read in the preceding time. Since time difference is used as a speed command as described above, the optical head keeps on moving at the same speed with a result that it tends to go past the target position and cannot reach it smoothly.